Mixed-species Plots Research Articles

Effects of initial spacing and species mix on stand developmental patterns in two field trials in central Oregon

Productivity, stand dynamics, and ecosystem processes characterizing mixed-species forests have been inferred from a mix of inventory data, observational studies, and designed experiments. Designed experiments are expensive to implement and maintain for a sufficiently long time in most temperate species, but associated treatment replication and randomization lower the risk of confounding from spatial correlations among species composition, environmental conditions, and productivity. Results from replicated mixed-species spacing trials in 45-yr-old Pinus ponderosa/Abies grandis plots and 53-yr-old P. ponderosa/P. contorta plots demonstrated main and interaction effects of spacing and species composition on growth and stand structure, as well as the change in these effects over time. Initial spacing ranged from 1.8 m to 5.5 m on a square grid. Mixtures of P. ponderosa and A. grandis at wide spacings offered A. grandis the opportunity to achieve cumulative height growth nearly equal to P. ponderosa and thereby form a uniform mixed canopy but lower cumulative stem volume growth. The same initial species mixture at closer spacing resulted in increasingly stratified mixtures with P. ponderosa dominating the overstory due largely to its more rapid juvenile height growth and wider crown, relegating shade tolerant A. grandis to the understory. A similar pattern emerged in the mixtures of P. ponderosa and P. contorta, but with more subtle gradients in vertical structure attributable to less extreme differences in juvenile height growth and shade tolerance. By the end of 2019 (plantation age 53), no significant overyielding or underyielding occurred in the mixed-species plots of P. ponderosa/P. contorta. In contrast, by the end of 2018 the mixed-species plots of P. ponderosa/A. grandis (plantation age 45 years) achieved some overyielding that was marginally significant (net volume production) or very significant (gross volume production) only at the narrowest (1.8-m) spacing. The timing and intensity of mortality shifted mixing effects on periodic annual increment slightly over time. Evidence for consistent overyielding among spacings was weak in these P. ponderosa/P. contorta and P. ponderosa/A. grandis spacing trials at total plantation age of 53 and 45 years, respectively. However, statistical power for detecting mixing effects was limited by the relatively small number of replications, and size differentiation continues to emerge at successively wider spacings on mixed-species plots.

Mixed-Species Gardens Increase Monarch Oviposition without Increasing Top-Down Predation.

Simple SummaryThe North American monarch butterfly is an iconic insect that has recently declined by over 80%, largely due to habitat loss. The primary approach to mitigate population declines is to plant milkweed, the primary host plant that monarch caterpillars feed and develop on. Recently, researchers have focused on optimizing monarch conservation habitats (i.e., milkweed plantings) in urban green spaces by studying habitat design and plant species selection. In many cases, as plant diversity increases, predatory and parasitic insect diversity increases and insect herbivore colonization and establishment decrease. We compared milkweed monocultures to a mixture of milkweed and other wildflower species to see what effects plant diversity have on monarchs and potential predators. We found that monarchs laid 22% more eggs on milkweed planted in mixed-species plots than milkweed in monoculture. We also found more predators in the mixed-species plantings, but this did not affect monarch disappearance rates. These results can be used to create evidence-based guidelines for monarch conservation habitats.Monarch butterfly populations have declined by over 80% in the last 20 years. Conservation efforts focus on the creation of milkweed habitats to mitigate this decline. Previous research has found monarchs lay more eggs per milkweed stem in urban gardens than natural habitats and recent work identified specific garden designs that make urban gardens more attractive to monarchs. Increasing plant diversity can reduce specialist insect herbivore colonization via bottom-up (e.g., plant) and top-down (e.g., predation) regulatory factors. Although this is beneficial for pest management efforts, it contradicts conservation efforts. In this study, we explored if adding multiple flowering species to garden-sized milkweed plantings affected monarch oviposition or top-down regulation of larvae. We compared monarch egg abundance, natural enemy abundance and richness, and biological control of monarch larvae in milkweed monocultures and milkweed mixed with four additional wildflower species. We found that monarchs laid 22% more eggs on sentinel milkweed plants in mixed-species plots with no effect of plant diversity on monarch survival. We also found higher natural enemy richness, wasp, and predatory bug abundance in the mixed-species plots and this did not translate to higher biological control rates. Our results provide more evidence that plant selection and habitat design are important for monarch conservation.

Effect of a tree mixture and water availability on soil nutrients and extracellular enzyme activities along the soil profile in an experimental forest

An increasing number of studies demonstrate that tree species biodiversity can affect primary productivity and nutrient cycling in forests due to several factors, such as complementarity, facilitation or selection effects. For instance, resource partitioning in soils has been found to allow a more optimized nutrient uptake in mixed species plots compared with monocultures. However, how these effects will be modified by climate change – such as water availability – is not as well understood, especially in deep soil layers. Therefore, we specifically asked how water availability may influence the effect of tree mixtures on soil microorganism activity by measuring extracellular enzyme activities (EEAs) and available nutrients along the soil profile (down to 90 cm) in a 10-year-old plantation in southwestern France, which manipulates tree species composition (Pinus pinaster and Betula pendula, in monocultures and in mixed plots) and irrigation. Our results showed that EEAs directly depend on tree species composition and water conditions in interaction with soil depth; we found a positive effect of mixing birch and pine on carbon and nitrogen-related EEAs at an intermediate soil depth (15–30 cm soil layer), while the effect of increasing water availability increased phosphorus-related EEAs mostly in the upper soil layers (0–30 cm). However, we found no significant interactive effect between tree diversity and water availability on EEAs, underlying that the negative effect of lower water availability cannot be offset by the positive effect of mixing tree species. Differences in EEAs and available nutrients along the soil profile highlight the importance to look at different soil depths to better understand how nutrient cycling may be affected by increasing tree diversity and changes in water availability.

Tree Growth and Wood Quality in Pure Vs. Mixed-Species Stands of European Beech and Calabrian Pine in Mediterranean Mountain Forests

Mixed-species forests may deliver more forest functions and services than monocultures, as being considered more resistant to disturbances than pure stands. However, information on wood quality in mixed-species vs. corresponding pure forests is poor. In this study, nine plots grouped into three triplets of pure and mixed-species stands of European beech and Calabrian pine (three dominated by European beech, three dominated by Calabrian pine, and three mixed-species plots) were analysed. We evaluated tree growth and wood quality through dendrochronological approaches and non-destructive technologies (acoustic detection), respectively, hypothesizing that the mixture might improve the fitness of each species and its wood quality. A linear mixed model was applied to test the effects of exogenous influences on the basal area index (BAI) and the dynamic modulus of elasticity (MOEd). The recruitment period (Rp) was studied to verify whether wood quality was independent from stem radial growth patterns. Results showed that the mixture effect influenced both wood quality and BAI. In the mixed-species plots, for each species, MOEd values were significantly higher than in the corresponding pure stands. The mixture effect aligned MOEd values, making wood quality uniform across the different diameter classes. In the mixed-species plots, a significant positive relationship between MOEd and Rp, but also significantly higher BAI values than in the pure plots, were found for European beech, but not for Calabrian pine. The results suggest the promotion of mixing of European beech and Calabrian pine in this harsh environment to potentially improve both tree growth and wood quality.

Species discrimination and individual tree detection for predicting main dendrometric characteristics in mixed temperate forests by use of airborne laser scanning and ultra-high-resolution imagery

This study aims to investigate the combined use of two types of remote sensing data — ALS derived and digital aerial photogrammetry data (based on imagery collected by airborne UAV sensors) — along with intensive field measurements for extracting and predicting tree and stand parameters in even-aged mixed forests. The study is located in South West Romania and analyzes data collected from mixed-species plots. The main tree species within each plot are Norway spruce (Picea abies L. Karst.) and Beech (Fagus sylvatica L.).The ALS data were used to extract the digital terrain model (DTM), digital surface model (DSM) and normalized canopy height model (CHM). Object-Based Image Analysis (OBIA) classification was performed to automatically detect and separate the main tree species. A local filtering algorithm with a canopy-height based variable window size was applied to identify the position, height and crown diameter of the main tree species within each plot. The filter was separately applied for each of the plots and for the areas covered with Norway spruce and beech trees, respectively (i.e. as resulted from OBIA classification).The dbh was predicted based on ALS data by statistical Monte Carlo simulations and a linear regression model that relates field dbh for each tree species with their corresponding ALS-derived tree height and crown diameter. The overall RMSE for each of the tree species within all the plots was 5.8 cm for the Norway spruce trees, respectively 5.9 cm for the beech trees. The results indicate a higher individual tree detection rate and subsequently a more precise estimation of dendrometric parameters for Norway spruce compared to beech trees located in spruce-beech even-aged mixed stands. Further investigations are required, particularly in the case of choosing the best method for individual tree detection of beech trees located in temperate even-aged mixed stands.

Modelling soil suction changes due to mixed species planting

Vegetation-induced changes of soil hydrology (i.e., infiltration rate and soil matric suction) has important implication to the stability of earthen infrastructure. Predicting the plant hydrological effects is challenging when multiple species are present. Most existing models consider only single-species due to lack of reliable and relevant experimental data and difficulty to partition solar radiant energy into individual species in a mixed-species condition. This paper aims to develop an improved hydrological model that can determine the water infiltration rate and suction responses under both single- and mixed-species (one tree and one grass species) condition. The model is verified by double-ring infiltration tests and field monitoring conducted in bare, grass-only, tree-only and mixed tree-grass plots. The model was subsequently used for parametric analyses. The mixed species plot had the lowest infiltration rate and preserved the greatest suctions because the tree-grass competition for water created high soil moisture deficit. Parametric study identifies a threshold ratio of tree and grass leaf area index of 2.0, beyond which the effects of grass root-water uptake on water competition with the adjacent tree were negligible. Induced suction in the mixed tree-grass plot was close to that in a tree-only plot. Contribution of tree root-water uptake to induced suction in a mixed species plot did not increase further when the leaf area index ratio exceeds 5.0.

Genetic diversity of two tropical tree species of the Dipterocarpaceae following logging and restoration in Borneo: high genetic diversity in plots with high species diversity

Background: The impact of logging and restoration on species diversity has been well studied in tropical forests. However, little is known about their effects on genetic diversity within species.Aims: We assess the degree of genetic diversity among dipterocarp seedlings used for enrichment planting of selectively logged forests in Sabah, Malaysia, and compare it with diversity in naturally regenerating seedlings.Methods: We sampled young leaf tissues from seedlings of Shorea leprosula and Parashorea malaanonan for DNA genotyping, using microsatellite markers.Results: The levels of genetic diversity (expected heterozygosity and rarefied allelic richness) of naturally regenerating seedlings were statistically indistinguishable among unlogged, once logged and repeatedly logged forest areas. Enrichment-planted seedlings of P. malaanonan exhibited similar levels of genetic diversity to naturally regenerating seedlings whereas those of S. leprosula had significantly lower genetic diversity than natural seedlings. Interestingly, reduction of genetic variation was consistently observed in single-species plots relative to mixed-species plots among enrichment-planted seedlings.Conclusions: There was no reduction of genetic variation in naturally regenerating dipterocarp seedlings in areas of selective logging. However, genetic variation of enrichment-planted seedlings was lower in single-species plots relative to mixed-species plots. This suggests that enrichment-planting strategies should adopt diverse mixtures that should promote levels of both species richness and genetic diversity within species.

Improved method for estimating tree crown diameter using high-resolution airborne data

IFER—Institute of Forest Ecosystem Research, Cs. armady 655,Jilove u Prahy 254 01, Czech RepublicAbstract. Automatic mapping of tree crown size (radius, diameter, or width) from remote sens-ing can provide a major benefit for practical and scientific purposes, but requires the develop-ment of accurate methods. This study presents an improved method for average tree crowndiameter estimation at a forest plot level from high-resolution airborne data. The improvedmethod consists of the combination of a window binarization procedure and a granulometricalgorithm, and avoids the complicated crown delineation procedure that is currently used toestimate crown size. The systematic error in average crown diameter estimates is correctedwith the improved method. The improved method is tested with coniferous, beech, andmixed-species forest plots based on airborne images of various spatial resolutions. The absolute(quantitative) accuracy of the improved crown diameter estimates is comparable or higher forboth monospecies plots and mixed-species plots than the current methods. The ability of theimproved method to produce good estimates for average crown diameters for monocultureand mixed species, to use remote sensing data of various spatial resolution and to operate inautomatic mode promisingly suggests its applicability to a wide range of forest systems.

Control of Phytolyma lata Walker (Scott.) Attack on Milicia excelsa (Welw.) C. C. Berg seedlings under plantation conditions

In this study, five Phytolyma pest control measures which included: a mono-species plot (protected by mesh netting: T1), mixed-species plot (Milicia + Terminalia), mixed-species plot with foliar chemical treatment (T3), mono-species with foliar chemical treatment (T4) and untreated mono-species plot of Milicia excelsa seedlings (T5), were set up in a completely randomized block experiment with four replications. The total height, collar diameter and number of leaves were monitored fortnightly, while the Relative Growth Rates (RGRs) for height and diameter were estimated after 24 weeks. Data were analysed using descriptive and inferential statistics. The treatment effects significantly influenced height, collar diameter and foliage production, but the block effects were negligible. The highest mean height was in T1 (104.35 ± 2.93 cm), while the lowest was in T5 (68.15 ± 5.37 cm). The RGR for height (median = 0.013 cm-1 week -1) and diameter (median = 0.024 cm -1 week cm-1) were highest for T1, while T5 had the lowest (0.005 cm cm-1 week-1 and 0.012 cm cm-1 week-1, respectively). The provision of a physical barrier (perimeter fence) proved to be the most effective management strategy to prevent Phytolyma infestation, while the mixed-species layout was a promising alternative.Keywords: Psyllid, Mixed-species plots, Iroko, Relative growth rate

Fungal disease incidence along tree diversity gradients depends on latitude in European forests.

European forests host a diversity of tree species that are increasingly threatened by fungal pathogens, which may have cascading consequences for forest ecosystems and their functioning. Previous experimental studies suggest that foliar and root pathogen abundance and disease severity decrease with increasing tree species diversity, but evidences from natural forests are rare. Here, we tested whether foliar fungal disease incidence was negatively affected by tree species diversity in different forest types across Europe. We measured the foliar fungal disease incidence on 16 different tree species in 209 plots in six European countries, representing a forest‐type gradient from the Mediterranean to boreal forests. Forest plots of single species (monoculture plots) and those with different combinations of two to five tree species (mixed species plots) were compared. Specifically, we analyzed the influence of tree species richness, functional type (conifer vs. broadleaved) and phylogenetic diversity on overall fungal disease incidence. The effect of tree species richness on disease incidence varied with latitude and functional type. Disease incidence tended to increase with tree diversity, in particular in northern latitudes. Disease incidence decreased with tree species richness in conifers, but not in broadleaved trees. However, for specific damage symptoms, no tree species richness effects were observed. Although the patterns were weak, susceptibility of forests to disease appears to depend on the forest site and tree type.

Tree water uptake in a tropical plantation varying in tree diversity: interspecific differences, seasonal shifts and complementarity

AbstractWe studied tree water uptake patterns, tested for complementary water use among species and analysed controlling factors in a tropical tree diversity experiment. The water uptake depth of five species was investigated across seasons and diversity levels using the natural abundance of water isotopes (δ2H, δ18O) and modelling. Three distinct water acquisition strategies were found for trees growing in monocultures during the dry season: strong reliance (>70%) on soil water from the upper layer (0–30 cm) (Cedrela odorata), uptake from the upper and deeper layers (>30 cm) in equal proportions (Hura crepitans, Anacardium excelsum and Luehea seemannii) and water uptake predominately from deeper layers (Tabebuia rosea). Seasonal shifts in water uptake were most pronounced for T. rosea. The water uptake pattern of a given species was independent of the diversity level underlining the importance of species identity and species characteristics in spatial and temporal tree water use. Statistics did not show a significant effect of diversity on source water fractions, but we did see some evidence for complementary water resource utilization in mixed species plots, especially in the dry season. Our results also demonstrated that the depth of soil water uptake was related to leaf phenology and tree transpiration rates. A higher proportion of water obtained from deeper soil layers was associated with a high percentage foliage cover in the dry season, which explained the higher transpiration rates. Copyright © 2014 John Wiley & Sons, Ltd.

Tree species identity has little impact on the structure of soil bacterial communities in a 10-year-old tropical tree plantation

We investigated whether tree species identity has a significant impact on the structure of soil bacterial communities in a tropical tree plantation (Sardinilla, Panama). The experimental site contains tree species native to Panama, planted in both monoculture and mixed-species plots. Using a DNA fingerprinting approach (automated ribosomal intergenic spacer analysis [ARISA]), we identified significant differences in the community structure of abundant bacterial taxa in the bulk soil among all monoculture plots. We similarly found differences among plots containing five, three, one or no tree species. While distance-based gradients in bacterial community structure were detected across the plantation, further investigation revealed that the observed heterogeneity was, in fact, poorly related to the tree species in a given plot. We provide evidence that site related features (e.g., variability in soil pH) play a more important role in regulating the structure of bacterial communities within the bulk soil than tree species identity or richness.

Effects of species richness, identity and environmental variables on growth in planted mangroves in Kenya

Anthropogenic impacts on biodiversity across a wide range of ecosystems are well docu- mented; however the responses of ecosystems to re- duced diversity are still poorly understood. We investi- gated the effects of species richness, species identity and environmental variables on aboveground biomass increment using replanted mangroves at Gazi Bay, Kenya. We planted 32 plots (36 m 2 ) with 8 treat - ments: all possible combinations of the trees Avicennia marina, Bruguiera gymnorrhiza, and Ceriops tagal and an unplanted control. Trees were planted in July and August 2004 and monitored annually until 2009. Growth was slow in the first 2 yr of the study, but by 2007 there was a significant treatment effect on aboveground biomass. A. marina showed strong competitive traits, with the best growth overall and enhanced growth of individual trees when planted in mixed species plots. The highest biomass was recorded in 3-species mixes; partitioning the net effects of species mixing showed a strong species selection effect, but there was also a complementarity effect in some of the three species plots. Biomass was positively correlated with presence of A. marina and negatively correlated with sediment salinity. We conclude that there is variation in the stages of plant development at which species richness effects manifest themselves; in addition the effects of environmental variables have a bearing on the nature and direction of the relationship between species richness and ecosystem function.

Preference of granivorous rodents for seeds of 12 temperate tree and shrub species used in direct sowing

Direct seeding is potentially a cost-effective method for creating woodland and is particularly suited to establishing mixtures of species. Seed predation can cause poor establishment, and information on granivore preference for seed of different species is essential to evaluate relative risk and the need for control measures. We investigated consistencies in the preference of small mammals for removing seeds of 12 tree and shrub species at three sites in southern England using freely available seeds presented as single or mixed species plots. Seed preferences were modelled as preference for removal of seeds of a particular species in relation to others present in a given time period. Across-site comparison of seed preference ranks showed significant agreement, especially at each extreme of the ranks. Quercus robur and Fagus sylvatica were always ranked first or second, followed by Corylus avellana and Acer pseudoplatanus, while Crataegus monogyna and Fraxinus excelsior were ranked last; the relative position of middle ranking species varied between tests. Removal of seeds of highly preferred species was usually absolute, whereas seeds of lower ranked species were more likely to be partially removed. For most species, preference coefficients were positively related to two explanatory variates: the logarithms of whole seed and coat dry mass. The exception was Fraxinus excelsior, where preference was lower than predicted, possibly because of chemical defences. Presentation of seeds as either single or mixed species plots had no overall effect on preference ranking. The results suggest that species that produce large nuts, are more vulnerable to predation-loss and might require additional measures, such as treatment with repellents, to reduce predation risk, or might, as in the case of Q. robur, be so attractive to rodents that it is unrealistic to direct sow them. The majority of the other species with smaller seed size, however, were much less susceptible to removal by predators. Any losses that do occur with these species are likely to be relatively small and tolerable, and localised.

Population‐level compensation impedes biological control of an invasive forb and indirect release of a native grass

The intentional introduction of specialist insect herbivores for biological control of exotic weeds provides ideal but understudied systems for evaluating important ecological concepts related to top-down control, plant compensatory responses, indirect effects, and the influence of environmental context on these processes. Centaurea stoebe (spotted knapweed) is a notorious rangeland weed that exhibited regional declines in the early 2000s, attributed to drought by some and to successful biocontrol by others. We initiated an experiment to quantify the effects of the biocontrol agent, Cyphocleonus achates, on Ce. stoebe and its interaction with a dominant native grass competitor, Pseudoroegneria spicata, under contrasting precipitation conditions. Plots containing monocultures of each plant species or equal mixtures of the two received factorial combinations of Cy. achates herbivory (exclusion or addition) and precipitation (May-June drought or "normal," defined by the 50-year average) for three years. Cy. achates herbivory reduced survival of adult Ce. stoebe plants by 9% overall, but this effect was stronger under normal precipitation compared to drought conditions, and stronger in mixed-species plots compared to monocultures. Herbivory had no effect on Ce. stoebe per capita seed production or on recruitment of seedlings or juveniles. In normal-precipitation plots of mixed composition, greater adult mortality due to Cy. achates herbivory resulted in increased recruitment of new adult Ce. stoebe. Due to this compensatory response to adult mortality, final Ce. stoebe densities did not differ between herbivory treatments regardless of context. Experimental drought reduced adult Ce. stoebe survival in mixed-species plots but did not impede recruitment of new adults or reduce final Ce. stoebe densities, perhaps due to the limited duration of the treatment. Ce. stoebe strongly depressed P. spicata reproduction and recruitment, but these impacts were not substantively alleviated by herbivory on Ce. stoebe. Population-level compensation by dominant plants may be an important factor inhibiting top-down effects in herbivore-driven and predator-driven cascades.

Deposition and canopy exchange processes in central-German beech forests differing in tree species diversity

Atmospheric deposition is an important nutrient input to forests. The chemical composition of the rainfall is altered by the forest canopy due to interception and canopy exchange. Bulk deposition and stand deposition (throughfall plus stemflow) of Na+, Cl−, K+, Ca2+, Mg2+, PO 4 3− , SO 4 2− , H+, Mn2+, Al3+, Fe2+, NH 4 + , NO 3 − and Norg were measured in nine deciduous forest plots with different tree species diversity in central Germany. Interception deposition and canopy exchange rates were calculated with a canopy budget model. The investigated forest plots were pure beech (Fagus sylvatica L.) plots, three-species plots (Fagus sylvatica, Tilia cordata Mill. or T. platyphyllos Scop. and Fraxinus excelsior L.) and five-species plots (Fagus sylvatica, T. cordata or T. platyphyllos, Fraxinus excelsior, Acer platanoides L., A. pseudoplatanus L. or A. campestre L. and Carpinus betulus L.). The interception deposition of all ions was highest in pure beech plots and was negatively related to the Shannon index. The stand deposition of K+, Ca2+, Mg2+ and PO 4 3− was higher in mixed species plots than in pure beech plots due to higher canopy leaching rates in the mixed species plots. The acid input to the canopy and to the soil was higher in pure beech plots than in mixed species plots. The high canopy leaching rates of Mn2+ in pure beech plots indicated differences in soil properties between the plot types. Indeed, pH, effective cation exchange capacity and base saturation were lower in pure beech plots. This may have contributed to the lower leaching rates of K+, Ca2+ and Mg2+ compared to the mixed species plots. However, foliar analyses indicated differences in the ion status among the tree species, which may additionally have influenced canopy exchange. In conclusion, the nutrient input to the soil resulting from deposition and canopy leaching was higher in mixed species plots than in pure beech plots, whereas the acid input was highest in pure beech plots.

Interspecific variation in leaf litter production, decomposition, and nitrogen and phosphorus loss from decomposing leaves in a humid subtropical forest ecosystem of northeastern India

Studies providing direct experimental evidence of species impact on litter dynamics in forest ecosystems are limited. The decomposition processes in subtropical forests are also poorly understood. We studied variation in quality and quantity of leaf litter production, decomposition, and N and P loss from decomposing foliar litter in three tree species as well as a mixed-species plot in a subtropical broad-leaved forest of northeastern India. The annual leaf litter production was highest in Rhododendron arboreum Sm. (7293 kg·ha–1·year–1) followed by Myrica esculenta Buch.-Ham. ex D. Don (6902 kg·ha–1·year–1), mixed plots (6808 kg·ha–1·year–1), and Neolitsea cassia (L.) Kosterm (6299 kg·ha–1·year–1). The annual N and P inputs through litter were highest in the mixed plot (N, 111.0 kg·ha–1·year–1; P, 4.8 kg·ha–1·year–1) and lowest in the Rhododendron plot (N, 65.6 kg·ha–1·year–1; P, 2.9 kg·ha–1·year–1). The highest decay rate was recorded for Neolitsea (k = 0.89) and lowest for Myrica (k = 0.53) litter. The rate of N loss was highest for Neolitsea (kN = 1.39) and lowest for Myrica (kN = 0.68) species, and P loss was in the order of mixed (kP = 1.02) > Neolitsea (kP = 0.88) > Rhododendron(kP = 0.84) > Myrica (kP = 0.62). Acid-insoluble residue, which indicates lignin content and P-related litter chemistry, were correlated with the differential decomposition rates and nutrient loss pattern among the species.

Neighborhood effects and size‐asymmetric competition in a tree plantation varying in diversity

A plantation of native trees was established in Panama in 2001 to study the relationship between biodiversity and ecosystem functioning. Five years later, mixed-species plots had experienced enhanced tree growth compared with monocultures. Searching for underlying mechanisms, we developed a neighborhood model isolating size and identity effects. We found that the size of neighbors is, by far, the largest source of variation in individual-tree diameter and height. Size-asymmetric competition appears as a structuring factor in the plantation. The relative growth rate of small trees was significantly lower than that of large trees, and their height and basal diameter were most variable. The 50 smallest trees of the plantation suffered a disproportionate amount of death, and the proportion of small trees was highest in monoculture. Increased biomass allocation to branches for trees growing in three-species plots suggests that competition for light might be taking place. Clearly, local neighborhood plays a central role in determining productivity, suggesting that scale needs to be incorporated in the theoretical development and analysis of biodiversity and ecosystem functioning.

An invasive dandelion unilaterally reduces the reproduction of a native congener through competition for pollination

The impact of invasive alien species on native species is of increasing global concern. Invasive plants have various negative effects on natives through competition; however, relatively little is known about competition for pollination. The relationship between Japanese native dandelions (Taraxacum spp.) and invasive congeners may be a typical case of such an interaction. For example, native dandelions are being replaced by invasive congeners, especially in urban and suburban areas of Japan. To explain this phenomenon, we hypothesized that when natives are mixed with attractive invasives, natives may suffer from reduced seed set because invasives deprive natives of pollinators or because pollinators frequently move between species, resulting in interspecific pollen transfer. To test this hypothesis, we studied the effect of the invasive dandelion T. officinale on the pollination and seed set of the native T. japonicum using artificial arrays of monospecific and mixed-species plots as well as natural populations. Taraxacum officinale attracted more pollinator visits, perhaps because it produced more nectar than T. japonicum. The number of pollinator visits to T. japonicum was reduced when the congeners were grown together, and pollinators moved frequently between the two species. The proportion of seed set for T. japonicum was reduced in the presence of T. officinale in both artificial arrays and natural populations. These results support our hypothesis that interspecific competition for pollination plays an important role in the recent replacement of native dandelions by invasive congeners in Japan. Because invasive dandelions are apomicts, negative effects are incurred only by sexual natives. Thus, this system can be recognized as a rare case of interspecific interaction through pollination.

Tree species distribution and its impact on soil properties, and nitrogen and phosphorus mineralization in a humid subtropical forest ecosystem of northeastern India

The impact of tree species on net N and P mineralization, and soil properties beneath their canopy were studied in a subtropical evergreen broad-leaved forest of northeastern India. Four types of experimental plots were identified based on tree species dominance. The first plot was dominated by Myrica esculenta Buch.-Ham. ex D. Don., the second by Rhododendron arboreum Sm., the third by Neolitsea cassia Koster., and the fourth was a mixed-species plot. Organic carbon content (3.11%) and NH4+-N concentration (11.40 μg·g–1) in the Rhododendron plot, pH (4.64) and total N content (0.89%) in the mixed plot, available P (5.16 μg·g–1) and NO3–-N (8.63 μg·g–1) concentrations in the Neolitsea plot were significantly higher (p < 0.001) than the other plots. All these soil parameters were lowest in the Myrica plot. The net N and P mineralization rates in an annual cycle across different plots ranged between 18.83 and 22.14 μg·g–1·month–1and between 4.54 and 5.87 μg·g–1·month–1, respectively. The flux varied significantly (p < 0.001) among the plots, the lowest and highest being in the Myrica plot and mixed plots, respectively. The differences in soil properties and in net N and P mineralization among different species plots were related to litter quality and yield of the respective species as well as soil microenvironment.